Conventional state-of-the-art fiber Bragg grating (FBG) interrogation systems are typically bulky and heavy bench-top instruments that are assembled from off-the-shelf fiber optic and optical components integrated with a signal electronics board into an instrument console. The most basic FBG interrogation system uses a high-resolution near-infrared spectrometer, such as a telecommunications grade optical spectrum analyzer, in conjunction with a broadband light source, such as a white light fiber source, to interrogate the status of the FBG sensors. This type of system, although very accurate in terms of determining the peak wavelength position of the grating, is relatively slow in terms of signal acquisition.
More sophisticated and faster scanning FBG interrogators use either tunable laser (swept wavelength) systems or scanning Etalon interferometers, such as the systems developed by Micron Optics (Atlanta, Ga.). The Micron Optics wavelength scanning systems come in a variety of models with sub-picometer peak wavelength resolution, broad-spectrum (˜80 nm) capability, but with a relatively slow-scan, data acquisition rate typically from 1 Hz to 250 Hz. FBG interrogators with faster data acquisition rates are available such as the FiberPro2 from Luna Innovations (Roanoke, Va.), operating at data sampling rates of 1 kHz; the HS-FOIS produced by AEDP (Lanham, Md.) with data rates of up to 3.5 kHz; the I*Sense systems produced by IFOS (Santa Clara, Calif.) with data rates of up to 5 kHz; and the FBG read-out systems from Blue Road Research (Gresham, Oreg.) with data rates of up to 2 MHz.
The earliest fiber Bragg grating sensor work was performed by Morey and Meltz (SPIE Distributed and Multiplexed Fiber Optic Sensors, Vol. 1586, 1991; SPIE Chemical, Biochemical, and Environmental Fiber Sensors III, Vol. 1587, p. 351, 1991) at United Technologies. Since those early days, United Technologies has abandoned the field and fiber Bragg grating technology has blossomed in other hands into a key technology for telecommunications and sensing. The U.S. Naval Research Laboratory (ISA 0227-7576/97/747-756, Orlando, Fla., 1997) has been a leading research organization in fiber Bragg gratings for sensing applications. Its personnel have been responsible for many advances in fiber Bragg grating technology, including on-line fiber grating writing, but their primary emphasis has been on developing multiplexing and detection methods. Eric Udd of Blue Road Research has been a leader in the field of fiber optic smart aircraft structures since the early 1970s by developing many fiber optic technologies including fiber sensor gratings that can be used for the NDE assessment of aircraft structures. (SPIE, Smart Structures and Materials, Vol. 3330, p. 12-16, 1998).
Other research groups in the field include one at Virginia Polytechnic (SPIE, Smart Structures and Materials, Vol. 3330, p. 231-236, 1998) associated with Luna Innovations, that primarily has been investigating photo-induced, long-period gratings. Long-period gratings have minimal back reflection and must be used in a transmission mode, so the detection system must be double-ended and therefore more complex. To date, virtually all published work on FBG structural sensor systems has been focused on the detection of static strains or low-frequency (<5 kHz) vibrations (1998 Pacific Northwest Fiber Optic Sensor Workshop, Udd, E., (Ed.) Session 3, Paper No. 2, May 1998). Although complex FBG sensor systems have been developed, they rely on the use of off-the-shelf instrumentation that is typically very expensive. As a result, these systems remain in use in laboratory settings. The simplest of the FBG sensors available on the market today is a single point strain detection system, which sells for $8500 by Blue Road Research.
Therefore, there is a great need for a combined fiber Bragg grating interrogation with transceiver system representing a new, highly-robust, and reliable, technology that can be used to accurately monitor the status of an array of distributed fiber optic Bragg grating sensors installed in critical infrastructures.